Expert Answers: Basement Wall Insulation

Finishing your basement? HANDY has the answer for insulating walls.

Q: I am planning to finish my basement. What’s the best way to insulate the walls and hang drywall? - Joe Pizzo, Ontario, NY

A: At the Energy & Environmental Building Association (EEBA) seminar called “Houses That Work,” the presenter addressed basement insulation. Although the best practice is to insulate the exterior of foundation walls and floors, that’s not always feasible when you’re remodeling (unless you’re willing to dig around the foundation). But there are a couple of effective options for insulating the interior wall of a basement. No matter what method you choose, the first step is to make sure the basement is dry and will stay dry. Repair the causes of water infiltration problems before you begin a basement remodeling project.

Before you insulate the walls, you should address the rim joists – likely the coldest area in the basement. Because they’re above grade, they are exposed to colder air than the below-grade foundation wall. Insulate and seal the rim-joist areas between the main-floor joists with spray-foam insulation or pieces of rigid closed-cell insulation that is sealed at the edges with caulk or duct mastic.

If space is tight and you can’t sacrifice the thickness of full 2x4 walls, attach a rigid foam-insulation product that features channels for framing, such as Owens Corning Insulpink. Then attach the wall framing and mold-resistant drywall over the insulation.

If you have the space, a better option (see illustration) is to attach rigid closed-cell insulation (at least 1-1/2 in. thick) directly to the foundation wall and seal the seams with mesh tape and duct mastic (available at home centers near the HVAC materials). Build a 2x4 wall over the closed-cell insulation and attach mold-resistant drywall to the studs. You can also fill the voids between the studs with fiberglass insulation, but the added R-value you gain might not be worth the potential mess if the insulation ever gets wet.

In all cases it’s important not to install a vapor barrier on the drywall side of the studs. This application can trap condensation between the vapor barrier and the foundation wall. The result can be a situation many insulation contractors referred to as the “wet-diaper” effect - saturated insulation behind the vapor barrier that won’t dry out and will likely lead to mold problems.

I recommend checking out seminars and publications by the EEBA for more information about building-science issues. The association publishes several informative reference books about insulation, water management and ventilation as well as four large region-specific builder’s guides. All of the books are filled with high-quality illustrations and photos. - HANDY